The present invention relates generally to amorphous ceramic materials derived from resinous organosilicon polymers and more particularly to a method for increasing the density and the elastic modulus of those materials, and to the resulting ceramic article.
Ceramic materials of the type in question are typically formed as fibers having a composition consisting essentially of silicon and at least one of nitrogen and carbon. These ceramic fibers are useful, for example, as internal reinforcing elements in composite, high temperature-resistant materials, e.g., materials made into jet engine parts. The ceramic fibers are typically manufactured in a process employing resinous organosilicon polymers as the starting material. The polymer typically contains silicon, hydrogen, nitrogen and carbon, with some oxygen present, usually as an impurity. The polymer may also contain certain additives, such as chlorine and various metallic elements.
The resinous organosilicon polymer is typically processed into a fiber by melting solid polymer and then melt spinning the molten polymer, employing a conventional spinning device called a spinnerette, to form one or more filaments or fibers which are gathered into a fiber tow, and the fiber tow is wound around a spool. A fiber tow is composed of parallel filaments or fibers, e.g. 100-500 filaments in some embodiments. The fiber in the tow is subjected to curing following which the cured fiber is pyrolyzed in an inert atmosphere at relatively high temperatures to produce the ceramic fiber described above.
A more detailed description of a process for forming a ceramic fiber from a resinous organosilicon polymer is contained in an article by LeGrow et al, "Ceramics from Hydridopolysilazane", Am. Ceram. Soc. Bull., 66 [2] 363-67 (1987), and the disclosure thereof is incorporated herein by reference.
Pyrolysis is typically conducted by heating up to a temperature in the range 1000.degree.-1400.degree. C. in an inert atmosphere composed of nitrogen or argon, for example. During pyrolysis, the material undergoes a change in composition, from a resinous organosilicon polymer to a ceramic material, due to a loss of some of the components of the material (principally hydrogen but also some silicon, carbon, nitrogen and oxygen), in the form of gas or vapors. There is also both a weight loss in the article undergoing pyrolysis and a substantial shrinkage of the material (e.g., about 70% by volume), so that the net result is an increase in density as a result of pyrolysis, e.g., from about 1 to about 2.3 g/cc. However, the interior of the material is not as closely packed as is desired, and there is some free volume or voids in the interior, which is undesirable from the standpoint of subsequent events, as will be further explained below. In other words, the ceramic material, as pyrolyzed, has a relatively low density compared to the theoretical value of that characteristic.
The resinous organosilicon polymer starting material is amorphous (i.e. non-crystalline), and the ceramic material produced by pyrolysis is also essentially amorphous.
As noted above, the ceramic material is usually produced in the form of a fiber tow which in turn is used, for example, as an internal reinforcing element in a composite or laminated material. The composite material may have an organic polymer matrix, a metal matrix or a ceramic matrix. In the case of composite materials having a ceramic matrix, they are manufactured and used at elevated temperatures, up to e.g. 1400.degree. C. and above, in a variety of atmospheres including air. For these high temperature applications, it is generally desirable to manufacture the ceramic fibers at temperatures above the intended use temperature. However, when one pyrolyzes the fiber at temperatures above 1400.degree. C. at atmospheric pressure in an atmosphere of nitrogen or argon, there is generally a loss of physical properties, particularly tensile strength, during pyrolysis. While this higher temperature processing is desirable to stabilize the resulting ceramic fiber during its subsequent use, the attendant loss of tensile strength is undesirable. Such a loss of strength occurs because there is internal erosion and surface erosion on the ceramic fiber. Erosion occurs because some of the nitrogen or other components in the ceramic material is driven off as a gas or a component of a gas. The relatively low density of the ceramic material, following pyrolysis, is believed to facilitate the escape of that gas.
Erosion occurs because of the following equilibrium reaction: EQU composition X.revreaction.composition Y plus gas Z,
wherein gas Z may be nitrogen, carbon monoxide or silicon monoxide, or mixtures of two or more of these gases; composition X is the composition of a ceramic article before it undergoes heating at the elevated temperature; and composition Y is the composition of the article after the loss of gas Z. At atmospheric pressure, temperatures above 1400.degree. C. result in the equilibrium reaction proceeding in the direction of producing composition Y plus gas Z.
As noted above, following pyrolysis, the ceramic material contains silicon and at least one of nitrogen and carbon. In relation to each other, the silicon and nitrogen (and/or carbon) are not present in stoichiometric amounts, but a loss of nitrogen or carbon or silicon in the manner described above brings the relative amounts of silicon and nitrogen (and/or carbon) closer to stoichiometric, in turn increasing the likelihood of crystal formation. Very small crystals are not a problem, but once they have formed, small crystals grow relatively rapidly under the temperature conditions described above, and larger crystals cause decreased strength. Therefore, it is desirable to maintain the amorphous, non-crystalline character of the ceramic material. Further, for many uses, an increased elastic modulus is desirable, along with the material's amorphous character.
Crystalline silicon nitride is often subjected to a sintering operation to increase its density. The process for doing so generally requires the presence of several percent of a metal oxide sintering aid, such as Y.sub.2 O.sub.3, MgO, Al.sub.2 O.sub.3 or mixtures thereof, and a sintering temperature of 1900.degree. C. or higher in the presence of superatmospheric nitrogen is generally required. In this connection, see P. Popper "Sintering of Silicon Nitride, A Review", in "Progress In Nitrogen Ceramics", F. L. Riley, editor, M. Nijnoff Publishers, Boston, 1983, pp. 187-210.